Modern hard disc drives are commonly used in a multitude of computer environments, ranging from super computers through notebook computers, to store large amounts of data in a form that can be made readily available to a user. Typically, a disc drive comprises one or more magnetic discs that are rotated by a spindle motor at a constant high speed. The surface of each disc is a data recording surface divided into a series of generally concentric recording tracks radially spaced across a band having an inner diameter and an outer diameter. Extending around the discs, the data tracks store data within the radial extent of the tracks on the disc surfaces in the form of magnetic flux transitions induced by an array of transducers, otherwise commonly called read/write heads. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
The read/write head includes an interactive element such as a magnetic transducer which senses the magnetic transitions on a selected data track to read the data stored on the track. Alternatively, the read/write head transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the track.
As is known in the art, each read/write head is mounted to a rotary actuator arm and is selectively positionable by the actuator arm over a selected data track of the disc to either read data from or write data to the selected data track. The read/write head includes a slider assembly having an air-bearing surface that causes the read/write head to fly above the disc surface. The air bearing is developed as a result of load forces applied to the read/write head by a load arm interacting with air currents that are produced by rotation of the disc.
Typically, a plurality of open-center discs and spacer rings are alternately stacked on the hub of a spindle motor. The hub, defining the core of the stack, serves to align the discs and spacer rings around a common centerline. Movement of the discs and spacer ring,s is typically constrained by placing the stack under a compressive load and maintaining the load by means of a clamp ring. Collectively the discs, spacer rings, clamp ring and spindle motor hub define a disc pack envelope or disc pack. The read/write heads mounted on a complementary stack of actuator arms, which compose an actuator assembly, commonly called an "E-block," accesses the surfaces of the stacked discs of the disc pack. The E-block also generally includes read/write head wires which conduct electrical signals from the read/write heads to a flex circuit which, in turn, conducts the electrical signals to a flex circuit connector. The connector in turn is mounted to a flex circuit mounting bracket, and the mounting bracket is mounted to a disc drive basedeck. External to the basedeck the flex circuit connector is secured to a printed circuit board assembly (PCB). For a general discussion of E-block assembly techniques, see U.S. Pat. No. 5,404,636 entitled METHOD OF ASSEMBLING A DISC DRIVE ACTUATOR issued Apr. 11, 1995 to Stefansky et al., assigned to the assignee of the present invention.
The head-disc assembly (HDA) of a disc drive is typically assembled in a clean room environment. The need for maintaining a clean room environment (free of contaminants of 0.3 micron and larger) is to ensure the head-disc interface remains unencumbered and damage free. The slightest damage to the surface of a disc or read/write head can result in a catastrophic failure of the disc drive. The primary causes of catastrophic failure, particularly read/write head crashes (a non-recoverable, catastrophic failure of the disc drive) are generally characterized as contamination, exposure to mechanically induced shock, and non-shock induced damage. The source of non-shock induced damage is typically traced to the assembly process, and generally stems from handling damage sustained by the disc drive during the assembly process.
Several factors that bear particularly on the problem of assembly process induced damage are the physical size of the disc drive, the spacing of the components, the recording densities sought to be achieved and the level of precision to be maintained during the assembly process. The high levels of precision required by the assembly process are necessary to attain the operational tolerances required by the disc drive. The rigorous operational tolerances are in response to market demands that have driven the need to decrease the physical size of disc drive while simultaneously increasing disc drive storage capacity and performance characteristics. Demands on disc drive mechanical components and assembly procedures have become increasingly more critical in order to support capability and size in the face of these new market demands. Part-to-part variation in critical functional attributes in the magnitude of a micro-inch can result in disc drive failures. Additionally, as disc drive designs continue to decrease in size, smaller read/write heads, thinner substrates, longer and thinner actuator arms, and thinner gimbal assemblies will continue to be incorporated into the drives, significantly increasing the need to improve the assembly processes to protect the read/write heads and discs from damage resulting from incidental contact between mating components. The aforementioned factors resultingly increase the difficulty of assembling disc drives. As the assembly process becomes more difficult, the need to invent new tools, methods, and control systems to deal with the emerging complexities pose unique problems in need of solutions.
Coupled with the size and performance improvement demands are further market requirements for ever-increasing fault free performance. In response to demands for enhanced reliability, some solutions have begun to emerge. Some disc drives have incorporated the use of ramp load technology. By incorporating ramp load technology the need to physically merge the E-block assembly with the disc pack during the assembly process is circumvented. The read/write heads are not loaded onto the media until after completion of assembly and the drives are spun-up for the first time. The improved performance is obtained by eliminating read/write head induced media damage, basically by insuring an air bearing is present prior to the read/write heads being loaded to the discs.
Ramp load technology is generally limited to smaller disc drive systems, namely sub 3.5 inch form factors, because those disc drives have relatively few discs so tolerance stack-ups do not become a major factor in the assembly process. Increases in disc diameter, coupled with increasing the number of discs in the disc pack, heighten the demands of maintaining the dimensional, mechanical and operational integrity between the E-block and the disc pack. Tolerance stack-ups become very critical in the assembly process and conformation of dimensional attributes of the disc pack and the E-block assembly must be made prior to any attempts in merging the two. Dependence on ramp load technology as the means to accomplish the head-disc merge for larger diameter, multiple surface disc packs would permit a number of E-block to disc pack interface mismatches to escape the process, resulting in sub-optimal performance or even failure of the product. Ramp load technology fails to provide the precision and repeatability required by larger and more complex disc drives.
The progression of continually decreasing disc thickness and disc spacing, together with increasing track density and increasing numbers of discs in the disc pack, has resulted in a demand for tools, methods and control systems of ever increasing sophistication. A result of the growth in demand for sophisticated assembling equipment has been that a decreasing number of assembly tasks involve direct operator intervention. Many of the tasks involved in modern methods are beyond the capability of operators to reliably and repeatably perform.
In addition to the difficulties faced in assembling modern, high capacity, complex disc drives, actual product performance requirements have dictated the need to develop new process technologies to ensure compliance with operating specifications. The primary factor driving more stringent demands on the mechanical components and the assembly process is the continually increasing areal densities and data transfer rates of the disc drives.
The continuing trend in the disc drive industry is to develop products with ever increasing areal densities, decreasing access times and increasing rotational disc pack speeds. These three factors, in combination, place greater demands on the ability of modern servo systems to control the position of read/write heads relative to data tracks. As track densities continue to increase, a significant problem that results is the ability to assemble HDAs nominally free from the effects caused by unequal load forces on the read/write heads, disc pack imbalance or one of the components of runout, velocity and acceleration (commonly referred to as RVA). The components of RVA are: disc runout (a measure of the motion of the disc along the longitudinal axis of the motor as it rotates); velocity (a measure of variations in linear speed of the disc pack across the surface of the disc) and acceleration (a measure of the relative flatness of the discs in the disc pack). By design, a disc drive typically has a discreet threshold level of resistance to withstand rotationally induced noise and instability, below which the servo system is not impaired. Also, a fixed range of load forces must be maintained on the read/write head to ensure proper fly height for data exchange. The primary manifestations of mechanically induced noise and instability are (1) vibration induced read/write head oscillation, (2) beat frequencies written into the servo signal at the servo write station and (3) non-repeatable runout. Oscillations are often introduced to the system via (1) deformations of the disc surface, (2) harmonics induced by disc pack imbalance, or (3) excessive surface accelerations encountered by the read/write head while flying on track or traversing the disc surface during track seeks. Verification of disc pack compliance to the RVA specifications is crucial to the overall quality and long term reliability of the product. To ensure RVA compliance, measurements are taken to determine: (1) the amount of runout present in the disc pack, (2) absence of concave or convex disc profile as well as absence of a wavy disc profile across the surface of the discs, and (3) absence of a wavy disc profile around each track circumference.
The foregoing measurements require sophisticated metrological instruments and techniques. The complexity of the measurements render them very difficult for an operator to perform, particularly at high assembly run rates. Specific problems arising out of operator executed or operator assisted measurements include the frequency of damage to the discs and inconsistent and/or inaccurate measurement results obtained from a manually based measurement process. Both component damage and measurement errors occur from operator inability to maintain a sufficiently close interface with the measurement instruments as is demanded by the measurement process and associated instruments.
Damage to disc surfaces can cause read/write head crashes, while disc packs not in compliance to the surface acceleration profiles are known to cause at least three distinct problems in disc drive performance. The first problem relates to disc drive response to a concave or convex disc surface. A concave surface causes the fly height of the read/write head to decrease. A decrease in fly height increases the signal to noise ratio during read-write functions, but increases the read/write head susceptibility to surface aspirates that disrupt the air bearing, causing the read/write head to lose flight stability. A convex surface causes the fly height of the read/write head to increase. An increase in fly height decreases the read/write head susceptibility to surface aspirates but also decreases the signal to noise ratio during read/write functions. A significant decrease in the signal to noise ratio can cause data errors and/or servo burst misreads which cause the disc drive to suspend operations.
The second problem arising from non-complying disc packs relates to the drive response to radially wavy profiles across the surface of the discs in the disc pack. A disc profile of this nature causes abrupt changes in the read/write head fly height during seek operations. Abrupt changes in fly height encountered during seek operations can send the read/write head into oscillation, causing the read/write head to miss or misread a track-crossing, resulting in an overshoot or undershoot of the seek track. Furthermore, an abrupt change in fly height during a seek operation can cause contact that damages the disc and/or the read/write head. In a worst case, the contact can be of an intensity that results in a read/write head crash.
The third problem caused from non-complying disc packs is similar in nature to the second problem as it also relates to the disc drive response to wavy profiles. However, the wavy profiles of concern for this problem are circumferentially wavy disc surface profiles. The problem that is encountered when a read/write head encounters a circumferentially wavy disc surface profile is read/write head oscillation following a seek operation or during a track following operation. As with the radially non-flat surface, the circumferentially non-flat surface causes abrupt chances in read/write head fly height inducing the same type of responses described above, i.e., flight instability, oscillation, disc contact, read/write head crashes and even loss of servo lock.
Typically, in a phase lock loop servo system, after each seek a settling time is required to allow for seek induced read/write head oscillation to dampen out and allow the read/write head to come on track. Read/write head instability often results in the disc drive having, an inability to read the information contained in its servo frame. If instability remains at the end of the allocated settle time, the disc drive will normally retry the function. After a set number of unsuccessful retries the disc drive reports a failure to the system and discontinues the seek process. However, should the system attain servo lock, a mechanically induced noise causing read/write head oscillation of sufficient duration will cause the system to lose its lock and malfunction.
The operating performance of the disc drive servo system is affected by mechanical factors beyond the effects of mechanically induced read/write head oscillation from disc surface anomalies. Beat frequencies written into the servo frames during the servo track writing, process can cause servo system failure to phase lock, to lock to an inappropriate signal, or to lose phase lock and fall off track. Beat frequencies are typically caused by bernelled bearings (flat spots on a bearing surface resulting from handling or assembly damage), or by disc pack imbalance. Mechanical noise can cause perceived amplitude changes in the servo burst signals through acceleration induced fly height changes. Shifts in servo burst signal amplitudes, perceived or real, cause the servo system to adjust the position of the read/write head. If the signals are false, the servo system can drive the read/write heads off track, causing the drive to halt operations. Additionally, mechanical noise can supply frequency response mis-queues to the servo system. The frequency response mis-queues are a result of harmonics being generated by the mechanics in the same frequency range as the servo system crossover frequency. Either phenomenon can cause the servo system to drive the read/write head off track.
Another form of mechanical noise induced malfunction of the servo system is runout. One intent of a disc drive design is to have nominally concentric data tracks. Concentricity of a data track is measured from the ideal or theoretical center of rotation of the disc pack. From the perspective of the read/write head, each data track is positioned a fixed distance from the theoretical center of rotation the disc pack. Servo systems are designed with this geometric relationship in mind. If the actual concentricity of the data track excessively or abruptly deviates from the theoretical concentricity the servo system will be incapable of responding with appropriate corrections to allow the servo system to maintain its phase lock, a condition required to assure that read/write heads stay on track.
A related problem that occurs as track densities increase is variation in the width of the tracks. Whereas such variations in track width have not been a significant factor in obtaining accurate servo control in previous disc drives having relatively lower track densities, as track densities continue to increase variations in track width become increasingly significant. Such variations in track width can occur as a result of imperfections in the magnetic media of the discs, or can occur as a result of errors in the servo track writing process during manufacturing. Errors are traceable to the same family of disc pack imbalance and RVA noise sources discussed hereinabove. Even with improved approaches to the generation of position error signals in the disc drive servo system, the ability of the system to deal with such issues is finite. The limits of the servo system capability to reliably control the position of the read/write head relative to the data track must not be consumed by the noise present in the HDA resulting from the assembly process. Consumption of the available margin by the assembly process leaves no margrin in the system to accommodate chances in the disc drive attributes over the life of the product. An inability to accommodate changes in the disc drive attributes leads to field failures and an overall loss in product reliability, a detrimental impact to product market position.
Although the servo system is the system primarily affected by mechanically induced system noise, the disc drive read-write channel is equally dependent upon the mechanical integrity of the HDA. The issues discussed hereinabove regarding the inability of an oscillating read/write head to accurately read servo data also applies to read-write data. However, it is typical for read-write data to demonstrate a much lower signal to noise ratio than is present in the disc drive servo burst signals and gray code, thereby rendering read/write head capability in read data fields more susceptible to read errors. Read errors have frequently been traced to head-disc misalignments of the type causing a change in the fly height characteristics of the read/write head. Changes in fly height that increase the fly height cause the read/write head transducer to be located farther away from the data fields. The increased distance between the transducer and the data field imparts the perception of a decrease in data bit field strength relative to the background noise, resulting in an inability to read the data contained in the data field. Attempts to perform accurate measurements of head-disc misalignments occurring as a result of disc pack tilt have not been successful in manual head-disc merge operations. The inability to verify the presence of a head-disc misalignment during the read/write head-disc merge operation leads to rework of disc drives that subsequently fail in the disc drive production process. Reworking of disc drives exposes the disc drive, in particular the disc drive HDA, to increased handling, thereby increasing, the probability of damage to the disc drive.
Components of modern disc drives have a relatively high susceptibility to damage induced through mechanical shock. One type of shock induced damage presented by prior merge operations deals with the problem of "head slap." Head slap is a term used to describe the dynamics of a read/write head, resting on a disc, in response to mechanically induced shock. The shock causes the read/write head to lift off the disc, and once off the disc the gimbal spring cants the read/write head as the force of the load arm drives the read/write head back to the disc. Typically, the first point of contact of the read/write head are the corners thereof against the disc surface. It is known that shocks of a load of greater than 20 grams for duration of 0.5 milliseconds or less will cause head slaps. It is also well known that the results of head slaps often lead to read/write head crashes.
Taken in combination--the tasks involved in assembling a modern disc drive exceeds the capability of manual assemblers; the susceptibility of the disc drive to damage during the assembly process; the level of precision assembly required by increasing areal densities; and the need to minimize adverse effects of mechanically induced noise on the disc drive servo system--have culminated to render prior disc drive assembly method archaic.
Thus, in general, there is a need for an improved approach to disc drive assembling technology to minimize the potential of damage during assembly, to produce product that is design compliant and reliable, and to minimize mechanically induced system noise. More particularly, there is a need for an automated dynamic balance correction of a disc drive.